1. Field of the Invention
This invention relates generally to control circuits for controlling the magnitude of an alternating current, and more particularly to alternating current control circuits utilizing a semiconductor device as a variable impedance element to an alternating current supplied to a load.
2. Description of the Prior Art
There have been proposed several alternating current control circuits for controlling manually or automatically the operation of apparatus which operates from an alternating current. Such apparatus may, for example, be an AC (alternating current) motor. In such circuits, a variable impedance device or circuit has been provided in the path of the alternating current supplied to the apparatus and the impedance value of the variable impedance device has been made variable to control the magnitude of the alternating current passing therethrough. Generally, it is required for the variable impedance device or circuit used for controlling the magnitude of the alternating current to have bidirectional conductivity with the same impedance value to current flow either direction. To comply with the requirement, it has been previously proposed to use a diode-bridge circuit having four arms, each arm including a diode, and one pair of opposed connection points each provided between the arms adjacent each other which are connected to the path of the alternating current supplied to the apparatus and the other pair of opposed connection points being provided between the arms adjacent each other which are connected respectively to, for example, a collector and an emitter of a transistor which is supplied with a control signal between its base and the emitter to change the impedance value between the collector and the emitter. Each half cycle of the alternate current supplied to the apparatus flows through the collector-emitter path of the transistor in the same direction as a result of passing through the diode-bridge circuit and therefore the magnitude of every half cycle of the alternative current can be controlled by the transistor in the same manner though the transistor is substantially a device of unidirectional conductivity.
However, in such previously proposed circuits mentioned above, there is the disadvantage that it is difficult to supply a controlled alternating current smoothly to the apparatus because the alternating current passing through the diode-bridge circuit is given a discontinuous wave form by the switching operation of the diode therein. In the case where the apparatus to be controlled is an AC motor, the supply of such an alternating current having the discontinuous wave form causes the AC motor to generate an undesirable spike noise. Furthermore, there is the additional disadvantage that a relatively large number of circuit elements are required.